Market demand, as well as governmental legislation, has led automotive manufacturers to continuously improve fuel economy and reduce CO2 emissions across engine families, while simultaneously maintaining performance (horsepower). Using smaller engines providing higher power densities, increasing boost pressure, by using turbochargers or superchargers to increase specific output and down-speeding the engine by using higher transmission gear ratios allowed by higher torque generation at lower engine speeds have allowed engine manufacturers to provide excellent, performance while reducing frictional and pumping losses. However, higher torque at lower engine speeds has been found to cause random pre-ignition in engines at low speeds, a phenomenon known as Low Speed Pre-Ignition, or LSPI, resulting in extremely high cylinder peak pressures, which can lead to catastrophic engine failure. The possibility of LSPI prevents engine manufacturers from fully optimizing engine torque at lower engine speed in such smaller, high-output engines.
While not wishing to be bound by any specific theory, it is believed that LSPI may be caused, at least in part, by auto-ignition of droplets, e.g. comprising engine oil, or a mixture of engine oil, fuel and/or deposits, that enter the engine combustion chamber from the piston crevice (space between the piston ring pack and cylinder liner) under high pressure, during periods in which the engine is operating at low speeds, and compression stroke time is longest (e.g., an engine having a 7.5 msec compression stroke at 4000 rpm may have a 24 msec compression stroke when operating at 1250 rpm). Therefore, it would be advantageous to identify and provide lubricating oil compositions that are resistant to auto-ignition and therefore prevent or ameliorate the occurrence of LSPI.
WO2015/42337 considers the use of ashless antioxidant additives for reducing LSPI events. WO2015/142340 considers the use of metal overbased detergents for reducing LSPI events. WO2015/4171980 relates to a method of reducing LSPI events by providing a boron-containing compound comprising a borated dispersant or a mixture of boron-containing compound and a non-borated dispersant.
The prior art has also recognised that reducing the calcium content of a lubricating oil formulation can lead to a reduction in LSPI events, see for example, EP 2940110. However, detergents are often considered to be necessary additives for maintaining basic engine oils performance. Thus, recent efforts in providing lubricating oil formulations that reduce LSPI events have focused on replacing calcium detergents with alternative detergents. Nevertheless, there remains a need for a lubricating oil composition suitable for use in modern direct injection-spark ignition engines that reduces occurrences of LSPI events.